Abstract/Summary

The Lewisian complex of NW Scotland is dominantly composed of Archaean tonalitic
to granodioritic gneisses, ultramafic bodies and minor metasedimentary components.
Although the area is internationally well known and has been much studied for over a
century, the recognition and precise timing of some high-grade metamorphic events
has proven difficult to ascertain. This is partly due to repeated deformational and
metamorphic episodes in the Palaeoproterozoic which overprint and obscure earlier
events. We present data from both laser ablation (LA) ICP-MS and an adaptation of a
U-Pb chemical abrasion ID-TIMS technique applied to multi-age component zircons
from the Assynt (“Central”) block of this region. The new data reveal a previously
unrecognised complexity and provide the first unequivocal proof of an Archean to
Paleoproterozoic granulite metamorphic event in the Assynt area.
LA-ICP-MS U-Pb dating has indicated a ca 2.8 Ga protolith age for a tonalitic gneiss
with evidence for a ca. 3.5 Ga xenocrystic component (the oldest discovered in the
UK). Non-conventional U-Pb ID-TIMS utilising a combination of high-temperature
annealing followed by multi-step incremental dissolution on single grains allows
identification of Pb-loss and multi-generational age trajectories on 206Pb/238U-
207Pb/235U plots. A combination of LA-ICP-MS and this non-conventional TIMS work
has dated zircon growth at ca 2.7 Ga (“Badcallian”) and 2.5 Ga (“Inverian”) with
later Pb-loss occurring at ca 1.9 Ga and ca 1.7 Ga (early and late “Laxfordian”
respectively). This TIMS method is unique in that it combines a pseudo-spatial
resolution normally associated with an in-situ technique but benefits from the highprecision
analysis required to differentiate between these metamorphic events at ca
2.7 and 2.5 Ga.
Zircon Hf isotopes indicate that some gneisses from the Assynt area are typical of
Archaean continental crust (epsilon Hf ca -1). The tonalite gneisses however have
strongly negative epsilon Hf values of -7 to -10 indicating a more complex history of
derivation through partial melting of ancient crust with residual garnet as a long-lived
control on Hf. Moreover, consistent zircon epsilon Hf values from inherited cores,
igneous overgrowths and two separate metamorphic events indicate that the tonalitic
gneisses were formed by crustal recycling, rather than new additions to the crust.
These events may be summarised as: zircon crystallisation from a magma at ca 3.5
Ga, partial melting and crustal recycling producing the tonalite gneiss protoliths at ca
2.8 Ga, a prolonged lower crustal residence in granulite P-T conditions by ca 2.7 Ga,
further metamorphism in amphibolite conditions at ca 2.5 Ga and later deformation
associated with punctuated terrane amalgamation events between ca 1.9 Ga and ca 1.7
Ga. The occurrence of a 2.7 Ga metamorphic event preserved in gneisses from Assynt
contradicts the assertion of some previous studies that it does not exist in this region
and suggests at least some local terrane amalgamation occurred in the Archean.